1. Field of the Invention
The present invention relates to a method for determining whether or not wheels are locked for use in an electric power steering control apparatus.
2. Description of the Related Art
An electric power steering control apparatus is one in which steering torque is detected by a torque detector and an auxiliary torque, approximately proportional to the output of the torque detector, is supplied from an electric motor to a steering shaft to assist the steering effort by reducing the force required to turn the steering wheel.
The electric power steering apparatus differs from conventional hydraulically assisted power steering in that the power necessary to assist the steering effort is provided using an electric motor, not by hydraulic pressure. The amount of assist is determined by detecting the amount of twist of the steering shaft and by controlling the motor so as to reduce the amount of twist below a predetermined value. Further, as an output comparable to that produced by hydraulic pressure has to be generated using an electric motor, the electric motor requires large power to operate, and the battery as the power source of the motor must have a capacity that can supply the necessary power to the motor.
It is known to provide an electric power steering control system in which an electromagnetic clutch is controlled with fine accuracy so as to prevent the steering wheel from becoming locked or extremely heavy, thereby preventing unnecessary slippage of the electromagnetic clutch as well as occurrence of noise and impact (for example, Japanese Unexamined Patent Publication No. H07-257415).
There is also known an electric power steering control system in which provisions are made to prevent an excessive current from flowing to the electric motor when the steering system is turned to its maximum steering angle position (for example, Japanese Unexamined Patent Publication No. H11-49014).
Because cost increases, above that of conventional hydraulic control systems, are not acceptable for electric power steering control systems, the cost of each component part is reduced. Accordingly, the power capacity of the motor drive circuit is reduced to the minimum required in order to reduce the cost. As a result, if the output is continued to be produced for a prolonged time when the wheel is in a locked state, the capability of the motor drive circuit will be exceeded. Since the purpose of the electric power steering system is to assist the steering effort, there is no need to provide the steering assist when the wheel is in a locked state or when the steering wheel is turned to the full-lock position.
Under normal conditions, on the other hand, as power assist is provided when the motor is energized, the amount of twist in the steering wheel should decrease. If the amount of twist does not decrease, this means that either the wheel is in a locked state or the steering wheel is turned to the full-lock position, and therefore, no power assist is needed in this condition. However, if the assist is abruptly stopped, the driver will feel unnatural because the steering wheel abruptly becomes heavy.
In view of this situation, it is an object of the present invention to provide a method for determining whether or not wheels are locked. Another object of the invention is to provide a method that takes appropriate measures when it is determined that the wheel is locked.